The invention relates to an ultrasonic imaging apparatus for ultrasonic-echo recording according to the impulse-echo-principle, comprising ultrasonic transmitter/receiver which delivers echo signals with a specified dynamic range, as well as comprising an image recording device for the echo signals which manifest a non-linear luminance/control-characteristic.
In addition to the surface contours of the organs and vessels, the ultrasonic imaging process also effects the representation of the fine structures within the organs and vessels. The quality of an ultrasonic image is not only determined by the physical point-resolution in the ultrasonic beam; on the contrary, it is also influenced by the type of amplitude reproduction on the viewing screen of the oscilloscope tube or on the photographic medium for a photographed visual image. Echo signals which differ in their amplitude are represented on the viewing screen as image points of varying luminance. Luminance differences are perceived correspondingly by the eye of the observer as differences in the grey value. The amplitudes of the echoes received from the interior of the body comprise a dynamic range of more than 100 dB ("maximum echo contrast"). The structure-echoes, occurring as the result of diffuse scattering, are here distinguished by their smaller amplitude from the contour echoes, largely produced by means of specular (or direct) reflection, which manifest, without exception, a greater amplitude. Contrasting with the echo dynamic range of approximately 100 dB, there is a dynamic range of the luminance of only a maximum of 30 dB which the adapted eye of the observer can resolve into grey values between black and white ("maximum image contrast"). According to the Weber-Fechner-law, the eye evaluates the luminance perceived within this range logarithmically. The smallest difference in luminance which still can be distinguished by the eye is the threshold contrast d.sub.0 ; it amounts to d.sub.0 =0.83 dB. Accordingly, the adapted eye of the observer resolves the 30 dB-range into approximately 35 grey steps. These sensory-physiological factors are of significance for ultrasonography in two respects. On the one hand, the eye can detect only a segment of the great echo dynamic range. This segment can be selected with the aid of a gain control, and the size (or magnitude) of the segment can be influenced by means of dynamic compression. On the other hand, the structure-resolution in the ultrasonic image is not solely determined by the physical lateral resolution defined as the half-width (width at one half maximum intensity) of the echo signal of a point-object; on the contrary, it is also influenced by the threshold contrast of the grey value resolution. Thus, e.g. two point-objects with the lateral spatial interval (or spacing) A in the ultrasonic image can be distinguished from one another as long as d.sub.1 -d.sub.2 is equal to or greater than n.multidot.d.sub.0, with d.sub.0 =0.83 dB, where d.sub.1 is the echo signal which results when one of the two objects is disposed on the beam axis, and d.sub.2 is the corresponding signal, given an interval A/2 of both objects from the beam axis; and where n is the compression factor for the instance of a dynamic compression of the form: luminance.about.(echo amplitude).sup.1/n. Instead of the (-6 dB)-width of the echo signal of a point object, the lateral resolution is accordingly given by the[-6-(n.multidot.0.83)]dB-width. This definition of the point resolution is only valid and invariant, e.g. with respect to the choice of the echo amplification, as long as the relationship between echo amplitude and luminance is linear, or if said relationship follows the above-indicated exponential function. The definition loses its validity when the transmission system follows a function deviating therefrom; for example, a logarithmic function.